The principle of optical triangulation is based on arranging a light transmitter and a spatially resolving light receiver mutually offset by a known basic distance. Transmission and reception light beam are then oriented at an angle to one another, which causes the reception light spot to move on the receiver in dependence on the distance to the detected object. The position of the reception light spot on the spatially resolving light receiver thus is a measure of the object distance.
There are not only measuring triangulation sensors, which determine and output a distance in the described manner, but also switching systems according to the triangulation principle whose switching behavior depends on the object distance. These sensors include background suppressing sensors. They are switching sensors, thus their output is only a binary object detection signal. At the same time, however, the construction of a triangulation sensor is utilized in order to generate two reception signals with a light receiver which is at least spatially resolving in a near and a far range. Their difference is evaluated with a switching threshold so as to limit the object detection to a certain distance range and to suppress reception signals outside this distance range as a background signal. A background suppressing sensor is for example disclosed in DE 197 21 105 C2, wherein switches are provided in order to assign the individual elements of a spatially resolving light receiver to the near or far range in a variable manner. DE 199 62 701 A1 describes a background suppressing sensor with a virtual separation element.
The relation between object distance and offset of the reception light spot on the light receiver is non-linear. Distance variations in the near range lead to large offsets of the reception light spot and only small offsets in the long range. At the same time, the reception light spot is not an ideal mathematical point and its extent also again depends on the object distance since the receiving optics cannot sharply image the entire distance range. These effects lead to measurement inaccuracies and, in the case of background suppressing sensors, to a switching point deviation and thus at least sometimes to faulty switching.
These problems are particularly serious with glossy objects, because the receiver pupil may then only be partially illuminated. There are sensors which are specifically configured for the detection of gloss. However, their goal is to detect the glossy object at all, or to determine its degree of gloss. A triangulating distance determination or background suppression is not provided and would not be solved by the means specifically introduced for the detection of gloss.
There are approaches in the prior art to improve the receiving optics in order to improve the linearity or to expand the measuring range for particularly near objects. For example, DE 102 20 037 C5 uses an additional near range lens which refracts the reception light the more towards the light transmitter the closer the object is. However, this only improves the behavior in the near range.
DE 10 2008 014 912 A1 arranges an additional correction lens between the receiver lens and the light receiver. As a result, the light beam focused by the receiver lens is selectively deflected in such a way that there is a greater spacing between the incident light beams on the light receiver and a sharp image. However, this is actually impossible with a correction lens affecting the entire reception beam path, but at most a partial improvement for a certain distance range is achieved.
Apart from triangulation, optical distance measurement is also possible in coaxial systems, for example by measuring the light time of flight. For the reception optics to deal with different distances, so-called multi-zone lenses are used, where concentric rings are responsible for certain distance ranges, as for example proposed in U.S. Pat. No. 5,347,137. Such a multi-zone lens with its symmetry is not useful in a triangulation sensor. Any gain in the near range would cause corresponding errors in the far range and vice versa.
The conventional solutions do not at all contribute to the aspect of partial illumination in the case of glossy objects.